In order to improve the performance of a photovoltaic device such as a solar battery, it is important to efficiently introduce sunlight into an inner portion of a substrate of the solar battery. Therefore, a light incident side surface of the substrate is subjected to texture processing, and hence light reflected on the surface is made incident on the surface again. Thus, more sunlight is introduced into the inner portion of the substrate to improve photoelectric conversion efficiency. The texture processing refers to processing for forming intentional fine unevenness having a size in a range of several tens nm to several tens μm in the surface of the substrate.
Anisotropic etching is widely employed as a method of forming a texture in a solar battery substrate in a case where the substrate is a single-crystalline substrate. The anisotropic etching depends on the crystal orientation and uses an alkali aqueous solution which contains sodium hydroxide or potassium hydroxide and has the dependence of etching rate on a crystal orientation. For example, when a surface of the substrate which is a (100) plane is anisotropically etched, a pyramid texture in which the (111) plane is exposed is formed.
However, when a polycrystalline substrate is employed as the substrate in the anisotropic etching method using the alkali aqueous solution, only a partial texture structure may be formed because the etching rate is significantly changed according to crystal planes and crystal plane orientations are not aligned. Therefore, there is a limit on reducing a reflectance, which leads to a problem. For example, a reflectance at a wavelength of 628 nm is approximately 36% in a case of silicon having a mirror-polished surface. When a single-crystalline silicon substrate with a (100) plane is wet-etched, the reflectance is approximately 15%. When a polycrystalline silicon substrate is wet-etched, the reflectance is approximately 27% to 30%.
Therefore, mixed acid etching using an etching-resistant mask has been proposed as a method of forming a texture structure in the entire surface without depending on crystal plane orientations. A lithography method used for a semiconductor process may be employed as an etching mask producing method, but the method is not suitable to manufacture a solar battery because a manufacturing cost is high. Thus, there has been proposed a method of mixing a fine particle having a low etching resistance into a solution containing an etching-resistant material and coating a substrate surface with the resultant mixture to produce the etching-resistant mask. Then, a texture is formed using fluoronitric acid capable of isotropic etching in different crystal plane orientations (see, for example, Patent Document 1).
Alternatively, there has also been proposed a method of directly forming a texture in a silicon substrate by laser processing (see, for example, Patent Document 2).
Patent Document 1: JP 2003-309276 A
Patent Document 2: JP 03-89518 A